This invention relates to a silver complex diffusion transfer process (DTR process) and more particularly, to a developer used in the process.
In the DTR process, the silver complex is imagewise transferred by diffusion from the silver halide emulsion layer to an image receptive layer where the silver complex is converted into a silver image mostly in the presence of physical development nuclei. For this purpose, layers are so disposed that the imagewise exposed silver halide emulsion layer may be in contact with the image receptive layer in the presence of a developing agent and a silver halide complexing agent, or, alternatively, such contact is established after exposure by bringing the former layer into contact with the latter layer, whereby the unexposed silver halide is converted into soluble silver complex. The silver halide in the exposed areas of silver halide emulsion layer is developed to metallic silver (chemical development) which is insoluble and non-diffusing. The silver halide in the unexposed areas is converted into soluble silver complex which is transferred to the image receptive layer and forms a silver image generally in the presence of development nuclei.
In the DTR process for the reproduction of documents or the preparation of block copy materials, it is important to form in the image receptive layer a silver image of high maximum density, high contrast and high sharpness. It is generally known that in order to obtain a transferred silver image of such quality, the speed of diffusion transfer development should be increased. Further, in the DTR process for the above purpose, it is also important that the developed silver image in the receptive layer be pure black or bluish black in color. It is known that to obtain such a transferred silver image, a so-called image toning agent (toner) should be added to the image receptive layer or during the diffusion transfer development.
Among various known toners, useful ones are nitrogen-containing heterocyclic compounds having a mercapto or thione group, particularly those which have a mercapto or thione group and, in addition, an alkyl group of 4 or more carbon atoms, or an aryl group, or a group in which both the alkyl and aryl groups are present (e.g. an aralkyl group). As a typical example of such compounds, mention may be made of 1-phenyl-5-mercaptotetrazole. Such a nitrogen-containing heterocyclic compound, however, has a disadvantage of markedly reducing the transfer development speed, though it acts as a toner superior to those nitrogen-containing heterocyclic compounds having no substituent or having as substituent a lower alkyl group or/and a group involving no carbon atoms (e.g. a halogen atom, hydroxyl group, and amino group). Because of its repressive effect on the speed of development, the nitrogen-containing heterocyclic compound is subject to restriction with respect to its concentration in the developer, resulting in not only insufficient tone improvement but also a silver image unsatisfactory in the levels of maximum density, contrast and sharpness, the improvement of which is achieved by high speed transfer development.